Can't enter pin No in Phone

I recently changed my contract from a pay as you go sim card to a monthly contract. I have two phones, a Nokia 5800 I bought brand new unlocked, and a Samsung Corby pro which I got free when I changed contracts. The Nokia worked fine with the PAYG sim but on the contract sim ( same card and pin), I tried to go back to the Nokia yesterday but when I boot the phone up I don’t get the “Enter Pin” screen. Anyone know why?